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Recent sedimentation rates are useful for quantifying how changes in a watershed affect soil erosion; however, typical geochronological methods for dating sediments are limited in temporal resolution, particularly for newly deposited sediments. We used sediment traps to measure short-term sediment accumulation rates in a natural oxbow lake whose watershed has a mix of agricultural and forested land. Precipitation data from a local Soil Climate Analysis Network (SCAN) site and water quality measurements from Beasley Lake were used to explain intra-annual patterns in sediment deposition. We found that sediment accumulation in the traps was highest in July and August, while the highest rate of runoff, indicated by increased precipitation and total suspended solids, occurred in late winter and early spring (February-April). Our results indicate that the trapping efficiency of Beasley Lake may be lower than expected due to timing of particle settling predominantly in the summer months. The rate and timing of sediment accumulation in Beasley lake was highly seasonal and occurred when water temperature and pH increased, potentially through interactions with algal blooms. Our findings contribute new understanding of the interactions between suspended sediments, algal biomass, and water chemistry in a natural oxbow lake and provide support for using sediment traps to measure intra-annual variability in sedimentation rates in oxbow lakes.

Supplemental irrigation is critical to ensure high and stable crop yields in many regions. Water needs for irrigation will increase in the future, because of higher demands for food, feed and biofuels, and warmer, more extreme, climates. Water withdrawals for irrigation has led to plummeting water tables in many aquifers. Water harvesting like on-farm ponds can be a more sustainable approach to meet water requirements for irrigation. Nevertheless, whether groundwater or on-farm pond is the most suitable source of irrigation water for a single farmer and the whole community depends not only on the crop water demands and the unpredictability of rainfall, but also on the farmers’ preferences, in terms of risk aversion and long/short-view orientation and how they evolve in time. Here we couple the dynamics of crop development to that of soil water availability and water stored in on-farm ponds and aquifers. For a community of farmers, we also consider each farmer’s short/long-view orientation, and how it evolves in time. While general, the model is applied to the case of the Lower Mississippi River Basin, in the south-eastern USA, where irrigation has already led to a significant decline in groundwater levels. Results show that, for a single farmer, production maximization and risk of low yield minimization are often irreconcilable criteria when sizing the on-farm pond. Moreover, on farm ponds as source of irrigation can be more advantageous as source of water for the community, leading to a higher and more stable income. Yet, this choice is beneficial for the individual farmer only under extreme climates and in communities where the use of on-farm ponds is common.

Agriculture in the United States must respond to escalating demands for productivity and efficiency, as well as pressures to improve its stewardship of natural resources. Growing global population and changing diets, combined with a greater societal awareness of agriculture's role in delivering ecosystem services beyond food, feed, fiber, and energy production, require a comprehensive perspective on where and how US agriculture can be sustainably intensified, that is, made more productive without exacerbating local and off-site environmental concerns. The USDA's Long-Term Agroecosystem Research (LTAR) network is composed of 18 locations distributed across the contiguous United States working together to integrate national and local agricultural priorities and advance the sustainable intensification of US agriculture. We explore here the concept of sustainable intensification as a framework for defining strategies to enhance production, environmental, and rural prosperity outcomes from agricultural systems. We also elucidate the diversity of factors that have shaped the past and present conditions of cropland, rangeland, and pastureland agroecosystems represented by the LTAR network and identify priorities for research in the areas of production, resource conservation and environmental quality, and rural prosperity. Ultimately, integrated long-term research on sustainable intensification at the national scale is critical to developing practices and programs that can anticipate and address challenges before they become crises.

Agricultural irrigation accounts for nearly 70% of global freshwater withdrawal. Among irrigation practices, contour-levee cascade irrigation is of particular interest as it is water-intensive and widely used in many rice production regions. Despite its significant environmental implications, no study has quantified the distribution of contour-levee irrigation. One major challenge of remote sensing-based contour-levee field detection is how to accurately identify the thin and curved levee lines whose appearance varies dramatically in different fields. This paper presents a new deep network-based method that jointly optimizes semantically meaningful features to quantify the contour-levee fields. This new method uses a bi-stream encoder-decoder architecture to capture spectral information and gradient features. To maintain image gradient sharpness, a skip connection approach is employed to facilitate gradient propagation across long-range connections. Moreover, the new method uses deep supervision to generate more informative features from the earlier hidden layers and superpixel segmentation to reduce classification noise as a post-processing step. By testing against 41 images across 10 Arkansas counties, the average accuracy was 86.23% and the method achieved 15%-17% improvement over benchmark methods. The results show that IrrNet-Bi-Seg maintains good transferability and is thus promising for larger-scale applications.

Many concepts have been proposed to explain hydrologic connectivity of hillslopes with streams. Hydrologic connectivity is most often defined by qualitative assessment of spatial patterns in perched water tables or soil moisture on hillslopes without a direct linkage to water flow from hillslopes to streams. This form of hydrologic connectivity may not explain the hydrologic response of catchments that have network(s) of preferential flow paths, e.g. soil pipes, which can provide intrinsic connectivity between hillslopes and streams. Duplex soils are known for developing perched water tables on hillslopes and fostering lateral flows, but the connectivity of localized perched water tables on hillslopes with soil pipes has not been fully established. The objectives of this study were to characterize pipeflow dynamics during storm events, the relationships between perched water tables on hillslopes and pipeflows, and their threshold behavior. Two well-characterized catchments in loess soil with a fragipan were selected for study because they contain multiple, laterally extensive (over 100 m) soil pipe networks. Hillslopes were instrumented with shallow wells adjacent to the soil pipes, and the wells and pipe collapse features were equipped with pressure transducers. Perched water tables developed on hillslopes during a wetting up period (October – December) and became well connected spatially across hillslope positions throughout the high flow period (January – March). The water table was not spatially connected on hillslopes during the drying out (April-June) and low flow (July-September) periods. Even when perched water tables were not well-connected, water flowing through soil pipes provided hydrologic connectivity between upper hillslopes and catchment outlets. Correlations between soil pipeflow and perched water tables depended on the size and location of soil pipes. The threshold relationship between available soil-moisture index plus storm precipitation (ASI + P) and pipeflow was dependent on the season and strongest during dry periods and not high flow seasons. This study demonstrated that soil pipes serve as a catchment backbone of preferential flow paths that provide intrinsic connectivity between upper hillslopes and streams.

Quantifying the effectiveness of erosion control practices in watersheds remains a difficult problem. Determination of recent sediment accumulation rates for lake sediments in agricultural watersheds using radioisotopes, such as lead-210 (210Pb) and cesium-137 (137Cs), is potentially a valuable means of assessing the effectiveness of soil conservation practices at the watershed scale. The predominance of sediment arriving in runoff from the watershed, variable sedimentation rates, and mechanical mixing of soil in nearby fields all present challenges in the conversion of radioisotope data to sedimentation rates. Four sediment cores from Beasley Lake, Mississippi, were used to demonstrate the application of the Constant Initial Concentration (CIC) model for calculating sediment age from the distribution of 210Pb bottom sediments. The activity of 137Cs was used to supplement the 210Pb data by providing a benchmark date within the core to calibrate the CIC model. Three of the four cores showed reductions in sediment accumulation rate within the 30 years prior to core collection in 2008 and 2011 by at least 50% relative to rates from before the adoption of soil conservation measures in the watershed. The most recently resolved rates were approximately 0.5 cm y−1 (0.2 in yr−1). The study demonstrates the application of the CIC model for developing sediment accumulation chronologies in agricultural catchments.

In many regions precipitation does not reliably meet crop water demands – a situation that climate change will likely exacerbate. Supplemental irrigation can help enhance and stabilize crop yields, but the need of water for irrigation has often led to groundwater over-exploitation. On-farm ponds can provide a more sustainable water source. Their use has often been promoted by local authorities, but, by converting a portion of cultivated area to water storage and reducing water availability downstream, on-farm ponds also imply constraints and trade-offs. For an effective exploitation of their potential benefits, they must be carefully designed and managed based on the local edaphic and climate conditions – a non trivial, task because of the cascading effects of rainfall unpredictability. Here we identify the most suitable on-farm pond size, according to two criteria: maximization of average yield (i.e., production maximization) and achievement of a minimum acceptable yield (i.e., risk minimization, accounting for the farmer’s risk aversion). To this aim, we develop a minimalist model, requiring few, physically based parameters, coupling crop biomass, soil moisture, and water stored in the pond. While general, the model is here applied to a case-study in the Lower Mississippi River Basin (USA). Simulations show that yield maximization and risk minimization are goals hard to reconcile, regardless of climatic conditions, soil type and irrigation strategy, with smaller ponds allowing the maximum average yield at the cost of reducing its stability from year to year. Stress avoidance irrigation ensures higher yields than deficit irrigation, even if it implies a faster use of the stored water. Future, more extreme climates will result in lower maximum average yields and narrower ranges of pond sizes ensuring desirable minimum yields.

Soil pipes are important subsurface flow pathways in many soil erosion phenomena. However, limited research has been performed on quantifying and characterizing their flow and transport characteristics. The objectives of this research were to determine the applicability of a streamflow model with transient storage in deriving flow and transport characteristics of soil pipes. Tracer data from pulse inputs were collected in four different soil pipes after a fluorescein dye was injected in the upstream end of each soil pipe network in three branches (west, middle, and east) of a main catchment and a back catchment in Goodwin Creek Experimental Watershed in Mississippi. Multiple sampling stations were positioned along each soil pipe network. The transient storage zone model OTIS-P was executed inversely to estimate transport parameters by soil pipe reach such as the soil pipe cross-sectional area (A), soil storage zone cross-sectional area (As), and exchange rate between the soil pipe and the soil storage zone (αs). Model convergence was achieved, and simulated breakthrough curves of the reaches were in good agreement with actual tracer data for eight of the nine reaches of the three branches of the Main Catchment and five of the seven reaches of the Back Catchment soil pipe. Simulation parameters for the soil pipe networks were similar to the range of values reported for flow and transport characteristics commonly observed in streams. Inversely, estimated soil pipe flow velocities were higher with increased tortuosity, which led to a smaller cross-sectional areas predicted for the soil pipe flowpaths, while other parameters were not sensitive to tortuosity. In general, application of One-Dimensional Transport with Inflow and Storage-P to this unique soil pipe condition suggested larger transient storage (As and αs) compared with most stream systems. This was hypothesized to be because of relatively higher ratio of the wetted perimeter to flow area in the soil pipe, the hydraulic roughness of the soil pipe, potential retention in collapsed portions of the pipe, and interaction with smaller preferential flow systems. Copyright © 2015 John Wiley & Sons, Ltd.

Concentrated surface runoff, such as associated with ephemeral gully channels, increases erosion and transfers fine sediment and associated agrichemicals from upland areas to stream channels. Ephemeral gully erosion on cropland may contribute up to 40 % or more of the sediment delivered to the edge of agricultural fields, significantly threatening the health of downstream ecological services. Typically, conservation practices developed for sheet and rill erosion are also expected to treat ephemeral gully erosion, but technology and tools are needed to account for the separate benefits and effects of practices on various sediment sources. Without improved research studies, subjective observations will continue to be used to satisfy quality criteria in lieu of scientifically defensible, quantitative methods to estimate the impact of gully erosion. Ephemeral gully channels evolve by one, or combination of, complex physical process in the form of incision, headcut migration upslope, and channel sidewalls expansion (widening). This study focused on the latter, ephemeral gully channel widening relationships. The impact of various width functions on erosion can be very significant and is dependent on discharge, slope, soil properties, and management conditions. A description is provided on six ephemeral gully widening relationships, followed by recommended improvements, comparative application, and identification of future research needs. Improvements in the development of ephemeral gully width algorithms are critical to understanding the impact of conservation practices on controlling ephemeral gully erosion. Tools are needed to predict and quantify ephemeral gully erosion, including the capability to evaluate the effect of conservation practices to control erosion. An improved critical shear stress equation was developed and described that provides an approach to incorporating impacts of management practices on the resulting gully erodibility. Conservation management planning by organizations needs a systematic approach to determining the extent of ephemeral gully erosion problems on a field, watershed, or national basis, or to predict the recurring or new locations of ephemeral gullies prior to their development.

The local role that land-atmosphere interactions play in the rainfall process has been often explored by investigating the initiation of moist convection as the top of the atmospheric boundary layer (ABL) crosses the lifting condensation level (LCL). However, this LCL crossing alone is not a sufficient indicator of the probability and intensity of subsequent convective precipitation, which is instead better characterized by the added consideration of the so-called convective available potential energy (CAPE). In this study, both the LCL crossing and CAPE are jointly considered as the primary indicators of the occurrence and intensity of moist convection in order to analyze the land-atmosphere interactions through a simple soil-plant system and a zero-dimensional mixed-layer model. The approach is explored using the free atmospheric conditions observed at the Central Facility in the Southern Great Plains, where the ABL analysis shows both dry and wet soil can be conducive to early moist convection depending on atmospheric conditions but CAPE always tends to be larger under wetter soil conditions. The combination of the two indicators, LCL crossing and CAPE, further allows us to classify free atmosphere and soil moisture regimes into positive and negative feedback regimes for moist convection.

Simplified numerical models of the atmospheric boundary layer (ABL) are useful both for understanding the underlying dynamics and potentially providing parsimonious modelling approaches for inclusion in larger models. Herein the governing equations of a simplified slab model of the uniformly mixed, purely convective, diurnal ABL are shown to allow immediate solutions for the potential temperature and specific humidity as functions of the ABL height and net radiation when expressed in integral form. By employing a linearized saturation vapour relation, the height of the mixed layer is shown to obey a non-linear ordinary differential equation with quadratic dependence on ABL height. A perturbation solution provides general analytical approximations, of which the leading term is shown to represent the contribution under equilibrium evaporation. These solutions allow the diurnal evolution of the height, potential temperature, and specific humidity (i.e., also vapour pressure deficit) of the mixed layer to be expressed analytically for arbitrary radiative forcing functions.

Little is known about the association of soil pipe collapse features with soil properties or land use history. Three loess covered catchments in northern Mississippi, USA were characterized to investigate these relationships. Soil pipe collapses were characterized for their size, type feature and spatial location along with soil properties across the three catchments. Although mapped as the same soil, one of the catchments did not contain pipe collapse features while the other two had 29.4 and 15.4 pipe collapses per hectare. These loess soils contained fragipan layers that are suspected of perching water, thereby initiating the piping processes. Pipe collapses associated with subsurface flow paths were not always consistent with surface topography. The surface layer tended to be non-erodible while layers below, even the upper fragipan layers, were susceptible to erosion by pipeflow. Soil properties of the lowest fragipan layer were highly variable but tended to prevent further downward erosion of soil pipes and thus formed a lower boundary for gullies. Middle to lower landscape positions in one of the piped catchments contained anthropic soils that were highly erodible. These anthropic soils were previously gullies that were filled-in in the 1950s when forested areas, assumed to have been established when land was previously converted from crop to forest land, were converted to pasture. Three decades after this land use change from forest to pasture, pipe collapses became evident. In contrast, the adjacent catchment that does not exhibit pipe collapse features experienced severe sheet and rill erosion prior to the 1930s while in cotton production. The surface horizons above the lower fragipan layer were completely removed during this period, thus the top-soil layer that tends to form a bridge above soil pipes in the more erodible subsoil layers was removed. This study showed that knowledge of soil characteristics or topography alone do not explain the distribution of soil pipe collapses as past land use can play a definitive role. Copyright © 2015 John Wiley & Sons, Ltd.

We experimentally investigate behavior in a bilateral oligopoly using a supply function equilibria model discussed by Klemperer and Meyer (1989), Hendricks and McAfee (2010), and Malueg and Yates (2009). We focus on the role that market size and the degree of firm heterogeneity have on the market equilibrium. Our results indicate that subjects within the experiment recognize the strategic incentives in a bilateral oligopoly, but they do not exploit these incentives to the exact magnitude predicted by theory. We find weaker support for predicted market outcomes, as market efficiency does not depend on market size, and in some cases buyers or sellers are more successful at extracting the rents from the market. (JEL L13, Q5, C9) I. INTRODUCTION Pollution permit markets have been applied to a wide variety of pollution problems. Recently, there has been quite a bit of interest in optimizing the scale of these markets (Wiliams 2003; Krysiak and Schweitzer 2010; Yates et al. 2013). When a market is optimized with respect to scale, the polluting firms are divided up into zones and firms may trade emissions only with other firms in their zone. In a given zone, there may be a small number of buyers and sellers, and if so, then all market participants in the zone, not just the buyers or just the sellers, exert market power. This market structure is called a bilateral oligopoly. In general, the strategic incentives inherent in bilateral oligopoly lead to a reduction in trade between firms relative to a competitive benchmark. For pollution permit markets, this reduction in trade has a cost (firms must expend more resources to reduce emissions) as well as a benefit (there are fewer localized damage hot spots). These effects tend to cancel out, implying that the bilateral oligopoly performs about as well overall as a competitive market (Yates et al. 2013). So a regulator need not be overly concerned if it turns out that a zone contains only a small number of firms, at least according to theory. Optimizing permit markets with respect to scale offers promise for the design of future permit markets. But regulators may be reluctant to put these insights into practice and create a zone with a small number of buyers and sellers without assurances that the resulting market will behave according to theoretical predictions. Toward that end, we conduct the first experimental test of a bilateral oligopoly utilizing a supply function equilibria market structure. Using varying market sizes and cost heterogeneity, we find that subjects within the experiment recognize the strategic incentives in a bilateral oligopoly. However, we find weaker support for predicted market outcomes as the overall market efficiency does not appear to depend on market size. Our cautious conclusion is that permit markets that are optimized for scale should generally perform as predicted. In comparison to other market structures (e.g., monopoly, monopsony, bilateral monopoly, oligopoly, or oligopsony), bilateral oligopoly has received surprisingly little attention. Virtually all of the extent literature is theoretical. Hendricks and McAfee (2010) use extensions of Klemperer and Meyer's (1989) supply function equilibria models to characterize bilateral oligopoly equilibria in a single market. Weretka (2011) uses similar techniques to analyze a general equilibrium setting. Bjornerstedt and Stennek (2001) do not use supply function equilibria, but rather analyze sequential bilateral bargaining between market participants in a network structure. Lange (2012), Malueg and Yates (2009), and Wirl (2009) have all studied bilateral oligopoly in the specific context of pollution permit markets. These papers are conceptually similar in that they all use techniques related to supply function equilibria. There are some differences in their exact structure, however, and here we focus on the model in Malueg and Yates (2009). The key feature of the Malueg and Yates (2009) model is a net-trade function, which specifies how many permits the firms are willing to buy and sell at various prices. …

We extend the analysis of optimal scale in pollution permit markets by allowing for both market power and private information. We characterize the total costs (abatement costs and damages) under market power and private information and compare them to total costs under competition. It is possible for both market power and private information to lead to lower total costs than competition, but generally the differences between the three market structures will be small. We also conduct an optimal scale analysis of nitrogen pollution from waste water treatment plants (WWTP) into North Carolina's Neuse River System. An economic model of damages and abatement costs is integrated with a hydro-ecological model of nitrogen flow through the Neuse. We determine the optimal number of trading zones and allocate the WWTP into these zones. Under the most likely regulatory scenario, we find cost savings of 1.55 million dollars per year under the optimal market design relative to the typical 303(d) regulation in which the WWTP are not allowed to trade.

Soil erosion, in its myriad forms, devastates arable land and infrastructure. As an integral landscape feature, gully erosion is a complex system as its evolution is controlled by upstream migration of a gully head (headcut face), incision of the gully bed (plunge pool) and gravitational mass-movement (widening) on gully channel sidewalls. Bed incision is often limited by the presence of a less erodible soil layer. When erosion reaches such a layer, the gully typically widens, creating a wide shallow cross section. Once a gully is initiated, transport and deposition of the eroded soil and widening of the channel further govern its evolution. Our knowledge of these processes within agricultural regions, however, is still quite limited with theory largely borrowed from river hydraulics. Experiments were conducted to examine gully expansion (widening) as a function of channel slope and overland flow discharge. Packed soil beds were subjected to simulated rainstorms followed by clear-water overland flow. As bed slope and discharge increased, channel widening increased. An empirical equation, based upon measured channel width from constant discharge experiments, was developed in terms of slope and discharge. By further quantifying key aspects of gully erosion processes, results from this experimental campaign will significantly improve soil erosion prediction technology for use in agricultural regions.

Experiments were conducted in a laboratory flume channel to evaluate the effects of increasing amounts of sand on its transport over and through an immobile coarse gravel bed. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges at approximately the same flow depth of 0.2 m for 11 different elevations of sand in the gravel bed. Sand transport was measured using both physical samples and a density cell. For a given flow rate, increases in the elevation of sand relative to gravel resulted in decreases of bed shear stress from 32–44% and increases in sand transport by three orders of magnitude. For the highest two discharges, the sand merged into a small number of long and low bed forms that translated through and over the gravel bed. A collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the cumulative probability distribution function of the gravel surface evaluated...

Networks of remotely monitored research sites are increasingly the tool used to study regional agricultural impacts on carbon and water fluxes. However, key national networks such as the National Ecological Observatory Network and AmeriFlux lack contributions from the Lower Mississippi River Basin (LMRB), a highly productive agricultural area with opportunities for soil carbon sequestration through conservation practices. The authors describe the rationale to create the new Delta-Flux network, which will coordinate efforts to quantify carbon and water budgets at seventeen eddy covariance flux tower sites in the LMRB. The network structure will facilitate climate-smart management strategies based on production-scale and continuous measurements of carbon and water fluxes from the landscape to the atmosphere under different soil and water management conditions. The seventeen instrumented field sites are expected to monitor fluxes within the most characteristic landscapes of the target area: row-crop fields, pasture, grasslands, forests, and marshes. The network participants are committed to open collaboration and efficient regionalization of site-level findings to support sustainable agricultural and forestry management and conservation of natural resources.

Sediment loads from gully erosion contribute to water quality problems, reduction in crop productivity by removal of nutrient-rich topsoil, and damage to downstream ecosystems. The identification of areas with high potential for gully channel development is often performed using spatially derived stream power estimates from second-order topographic indices, such as the compound topographic index (CTI). The utilization of CTI to identify where gullies develop is affected by field and local topographic characteristics and DEM resolution. In this study, the effect of overall terrain slope, local relief variance, and raster grid cell size on CTI cumulative distribution values was investigated using theoretical and observed catchment methodology. In the theoretical analysis, stochastic methods were used to generate simulated catchments to quantify the influence of overall terrain slope, local relief variance, and raster grid cell size (each considered individually). The observed methodology used three sites with distinct topographic characteristics, measured gully channels, and high-resolution topographic information. Raster grids for the three observed study sites were generated at varying raster grid cell sizes. Critical CTI values were determined through comparison of measured gully thalwegs with threshold CTI raster grids of the observed watersheds at different resolutions. Results from the theoretical investigation indicate that CTI values were linearly influenced by changes in relief variance and overall slope, while variations in raster grid cell size caused an inverse power variation in CTI values. In addition, variations in raster grid cell size, produced changes in cumulative distributions of the top 0.1% CTI values. The use of normalized CTI values (CTI n ) produced merged cumulative distribution curves when varying overall slope, terrain relief variance, and to a lesser degree DEM resolution. Similar findings were obtained from the analysis of observed catchments. When DEM resolution varied, the differences in critical CTI n values in the same field were significantly reduced when compared to original critical CTI values, although differences were not fully eliminated. Normalization of the CTI cumulative distributions improved comparisons between different sites with distinct drainage area sizes and topographic characteristics, providing a possible alternative for investigations of large watersheds with more than one topographic characteristic. Results suggest that a normalized critical CTI between 1 and 2 could be used for the identification of areas with high potential for gully development. Knowing where gullies develop is important in understanding the effect of conservation practices on soil erosion through the use of field-scale and watershed-scale simulation models. Effective watershed management plans depend on this information to target the placement of conservation practices for the efficient use of available resources.

Shortly after peak stage of the 2011 Mississippi River flood, water samples were collected from the river, from sand boils near the toe of the levee, and from actively flowing relief wells over a 55 km stretch north of Vicksburg, MS. Two distinct pathways for seepage under the levee were identified based on the elemental composition of water samples. Sand boil discharge was similar to water from relief wells only at a location where the levee sits on ancient channel fill deposits. Seepage at this site is forced along a deeper pathway beneath the fine-grained channel fill. Where the levee sits on sandy point bar deposits, shallow flow beneath the levee is unimpeded. The chemical composition of discharge from sand boils at these sites was clearly not just river water, nor a simple mixture of river and groundwater, but appears to reflect unique weathering or redox interactions occurring within the upper portion of the alluvial aquifer. Distinguishing shallow and deep seepage pathways may prove useful for evaluating site specific risk of levee failure.

Bed load is part of the total sediment load of a stream, comprising the coarsest fraction of mobile material that is frequently in contact with the stream bed. Sediment conveyed downstream may fill reservoirs and channels, impeding navigation, increasing the likelihood of flooding, and degrading water quality and aquatic habitats. Local erosion and deposition of bed material may also cause instability of channel banks. Any long-term program of channel stabilization or improvement must account for bed-load transport in order to avoid channel aggradation or degradation. The determination of bed load rates has been concentrated in four general methods (Hubbell 1964): (1) direct measurement by collecting physical samples, (2) using bed-load transport relations, (3) measuring the erosion or deposition of bed material sediment in a confined area, or (4) measuring bed-load transport indirectly using surrogate technologies. None of these techniques is suitable for a wide range of uses. No single bed-load transport relation has been shown to have general applicability (e.g., Gomez and Church 1989; Vanoni 2006, p. 221–222; Scheer et al. 2002). Many streams do not have a convenient area to carry out erosion or deposition measurements. Accurate determination of bed load using direct measurement methods is difficult because the rate and size of sediment in transport varies dramatically in time at a point and spatially at a given time over a channel cross section of a channel even when the flow is steady (Ehrenberger 1932; Hubbell et al. 1985; Whiting et al. 1988; Kuhnle and Southard 1988; Kuhnle et al. 1989; Gray et al. 1991; Bunte and Abt 2005). Matters are further complicated by unsteady flows. Designing a device that will sample with an equal efficiency over widely varying transport rates is itself a significant challenge. An additional challenge is to collect enough samples at a point and in a cross section to adequately define the mean rate of bed-load transport for a given flow strength. Recent improvements in technology have allowed the development of devices that do not collect physical samples of the sediment but collect indirect or surrogate information that is directly related to the transport of bed-load sediment. These surrogate methods have been divided into active and passive types by Gray et al. (2010), depending on whether the sensors emit signals and record properties of the reflected sound or passively record naturally generated sound emissions. Examples of active sensors consist of acoustic Doppler current profilers that track motion of the bed sediment (Rennie et al. 2002) and active tracking of tagged sediment particles (Emmett et al. 1996; Nichols 2004). Examples of passive sensors include impact pipes (Papanicolaou et al. 2009; Mizuyama et al. 2010), impact plates (Krein et al. 2008; Moen et al. 2010; Rickenmann et al. 2014), geophones (Tsakiris et al. 2014), and devices capable of sensing sediment-generated noise (SGN) associated with the transport of gravel and coarser sediment (Thorne 1986a; Barton et al. 2010). The following paragraphs will focus on passive acoustic techniques for measuring SGN using hydrophones deployed in the transporting flow. Bed load made up of gravel (diameter >2 mm) and larger particles has been shown in previous studies to generate sound through particle collisions and bed collisions, the amplitude of which is related to the particle transport rate (e.g., Thorne 1985, 1986a; Barton 2006). Passive acoustic instrumentation has several characteristics that make it attractive for use in measuring bed load. It is well suited for remote deployment because the hardware needed to sense and record sound is relatively inexpensive. Passive acoustic methods, which record sound that is generated at some distance from a hydrophone, is nonintrusive so the phenomenon being measured is minimally affected by the measurement process. Passive acoustic instrumentation integrates sound from a finite area, decreasing bias caused by spatial heterogeneity of bed-load transport. Continuous monitoring of bed-load transport in a minimally invasive manner is readily accomplished during infrequent high-magnitude events when coarse gravel and cobble sediments are in motion. These events are infrequent but have been shown to be responsible for transporting the majority of bed-load sediment in some streams (Kuhnle 1992). Challenges of using passive acoustics to measure bed load include the generally unknown size of the measurement volume and the acoustics of fluvial environments. This paper reviews the current state of knowledge supporting the use of passive acoustic technology for bed load monitoring using hydrophones deployed in the transporting flow to directly record interparticle collisions. It is shown that while prior SGN work has addressed the generation and characteristics of SGN signals and qualitative/empirical approaches to quantifying bed load using SGN in field settings, a lack of work to date on the propagation of SGN signals in the riverine environment limits SGN as a broadly applicable methodology. In the following section, the concept of using SGN to measure bed load is introduced, along with the development of the method to date. The acoustic characteristics of the riverine setting are then presented, including propagation, transmission losses, and boundary effects. Since sediment transport may not be the most dominant source of underwater sound in a river, the

Voltage clamping is an important tool for measuring individual currents from an electrically active cell. However, it is difficult to isolate individual currents without pharmacological or voltage inhibition. Herein, we present a technique that involves inserting a noise function into a standard voltage step protocol, which allows one to characterize the unique frequency response of an ion channel at different step potentials. Specifically, we compute the fast Fourier transform for a family of current traces at different step potentials for the inward rectifying potassium channel, K(ir)2.1, and the channel encoding the cardiac fast sodium current, Na(v)1.5. Each individual frequency magnitude, as a function of voltage step, is correlated to the peak current produced by each channel. The correlation coefficient vs. frequency relationship reveals that these two channels are associated with some unique frequencies with high absolute correlation. The individual IV relationship can then be recreated using only the unique frequencies with magnitudes of high absolute correlation. Thus, this study demonstrates that ion channels may exhibit unique frequency responses. Published version

While rainfall intermittency is a dynamical phenomenon, little progress has been made in the literature on the link between rainfall intermittency and atmospheric dynamics. We present the basic dynamical models of intermittency that are phenomenologically most similar to rainfall: Pomeau–Manneville Type-III and On–Off. We then illustrate each type with both a 1-D iterative map and a corresponding stochastic process stressing the appearance of these dynamics in high-dimensional (stochastic) systems as opposed to low-dimensional chaotic systems. We show that the pdf of rainfall intensities, the pdf of “laminar phases” (periods of zero rainfall intensity), and the spectrum of the rainfall series all have power-law behavior that is broadly consistent with intermittency in the classic types. Using a seasonal analysis, we find that summer convective rainfall at daily and sub-daily scales seems consistent with features of Type-III intermittency. The correspondence with Type-III intermittency and a preliminary entropic analysis further suggest that rainfall may be an example of sporadic randomness, blending deterministic and stochastic components.

The long-term success and sustainability of agriculture in the Lower Mississippi River Basin will depend largely on water resources. Aquifer decline in the region has been documented since the 1980s and continues today. Artificial recharge is one possible tool that could help alleviate this decline. This study describes a novel test of artificial recharge through direct well injection at a site in western Poinsett County, Arkansas. Water was taken from a source well 800 m (0.5 mi) from the injection well at a nearly uninterrupted rate of 565 gallons per minute for four days. A maximum rise in water level of 11.3 m (37.2 ft) at the injection well and 0.2 m (0.7 ft) at a well 100 m (330 ft) away from the injection well was recorded. Air entrainment was not extensive, particularly given that the recovery water-level data resulted in hydraulic conductivity values that were as large or larger than other values measured for the alluvial aquifer in the area. The success of artificial recharge hinges on the water quality of the source water. Water quality was collected monthly at five local reservoirs and the primary ditch used to fill each reservoir from January through April 2014. A reduction in turbidity was measured in the reservoirs compared to ditches. Water quality conditions in February were most conducive among the four months studied (January to April), and suggest this period as the most effective period to attempt artificial recharge in this area. However, more data are required to sufficiently characterize and strengthen these initial conclusions.

Determining the properties of starbursts requires spectral diagnostics of their ultraviolet radiation fields, to test whether very massive stars are present. We test several such diagnostics, using new models of line ratio behavior combining Cloudy, Starburst99 and up-to-date spectral atlases. For six galaxies we obtain new measurements of HeI 1.7 um / Brackett 10, a difficult to measure but physically simple (and therefore reliable) diagnostic. We obtain new measurements of HeI 2.06 um / Brackett gamma in five galaxies, and find that it and [OIII]/Hbeta are generally unreliable diagnostics in starbursts. The heteronuclear and homonuclear mid--infrared line ratios (notably [NeIII] 15.6 um / [NeII] 12.8 um) consistently agree with each other and with HeI 1.7 um / Br10; this argues that the mid-infrared line ratios are reliable diagnostics of spectral hardness. In a sample of 27 starbursts, [NeIII]/[NeII] ratios are significantly less excited than model predictions for a Salpeter IMF extending to 100 solar masses. Plausible model alterations strengthen this conclusion. By contrast, the low-mass and low-metallicity galaxies II Zw 40 and NGC 5253 show relatively high neon line ratios, compatible with a Salpeter slope extending to at least 40--60 Msol. One solution for the low neon line ratios in the high--metallicity starbursts would be that they are deficient in >40 Msol stars compared to a Salpeter IMF. An alternative explanation, which we prefer, is that massive stars in high--metallicity starbursts spend much of their lives embedded within ultra--compact HII regions that prevent the near- and mid-infrared nebular lines from forming and escaping. This hypothesis has important consequences for starburst modelling and interpretation.

Improving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent f...

[1] While both the mean and variance of daily temperature are forecasted to increase in future climate scenarios, studies of spring frost risk to vegetation have relied on changes in mean temperature to understand frost risk in these scenarios. We present a probabilistic model of spring frost risk based on the stochastic-crossing properties of a coupled temperature-phenology model in which the mean, variance, and autocorrelation structure of spring temperature may be controlled through independent parameters. The model results show that frost risk to vegetation is as sensitive to increases in daily temperature variance (which increases frost risk) as to increases in the mean temperature (which decreases frost risk).

The relative entropy between the joint probability distribution of backward and forward sequences is used to quantify time asymmetry (or irreversibility) for stationary time series. The parallel with the thermodynamic theory of nonequilibrium steady states allows us to link the degree of asymmetry in the time signal with the distance from equilibrium and the lack of detailed balance among its states. We study the statistics of time asymmetry in terms of the fluctuation theorem, showing that this type of relationship derives from simple general symmetries valid for any stationary time series.

Prior research has demonstrated the utility of metrics based on spatial velocity gradients to characterize and describe stream habitat, with higher gradients generally indicative of higher levels of physical heterogeneity and thus habitat quality. However, detailed velocity data needed to compute these metrics are difficult to obtain. Acoustic Doppler current profilers (ADCP) may be used to rapidly collect detailed representations of river velocity fields. Herein we demonstrate use of ADCP to obtain ecologically relevant data and compute associated metrics. Data were collected from four reaches of the Little Tallahatchie River in northern Mississippi. Sampled reaches were selected to observe velocity regimes associated with three distinctly different conditions: downstream from a major flow obstruction (a low weir), downstream from the apices of each of two bends, and within an extremely long, straight reach created by channelization. Three-dimensional velocity data sets from each site were used to compute metrics of habitat quality proposed by others. A habitat metric based on the presence of rotational flow in the vertical plane proved to be the best discriminator among conditions within the sampled reaches. Two of four habitat quality metrics computed from these measured velocities were greatest for the sharpest meander bend. ADCP hold great potential for study of riverine physical aquatic habitats, particularly at the reach scale. Additional work is needed to develop generally applicable field protocols and data reduction tools. Specifically, guidelines for ADCP settings and configuration appropriate for a range of riverine site conditions must be developed. Advances in instrumentation are needed to allow collection of information in closer proximity to the free surface and solid boundaries.

SummaryRolled beeswax foundation of varying mass and number of cells per dm2 was cut in three cell orientations, and tested mechanically in tensile and bending modes. The relative strength of the foundation was positively correlated with mass/dm2. The relative tensile strength, tensile stiffness, bending strength, and bending stiffness, were significantly greater in foundation oriented with cell walls horizontal instead of vertical. Comb-in-frame strength could therefore be enhanced considerably simply by changing the orientation of beeswax foundation sheets in conventional frames from the customary vertical orientation to a horizontal one.